FIG. 7 shows a structure of a thermal head as an example of a printhead which is used as a recording device for a facsimile machine, other office automation machine or the like, wherein a heating resistor is selectively heated to thermally cause a thermosensitive paper, which is fed in contact with the thermal head, to make a change in color for recording. Specifically, a printhead substrate 51 made of alumina ceramic for example is formed with a linear heating resistor 52, a common electrode 53 including comb-like teeth each having a tip in contact with the heating resistor 52, and a multiplicity of individual electrodes 54 each having one end in contact with the heating resistor 52 between adjacent teeth of the common electrode 53. Further, the printhead substrate 51 carries a plurality of drive ICs 55 attached thereon by die bonding. The drive ICs 55 are wire-bonded to the individual electrodes 52 each at the other end thereof while also wire-bonded to a multiplicity of conductive connecting portions 57 formed on a printed circuit board 56.
As shown in FIG. 8, a conventional drive IC 55 for driving a recording element, such as a heating resistor 52, of a printer is generally rectangular as viewed from above, and has a pair of longitudinal edges 55a, 55b. A multiplicity of output pads 61 are disposed in a staggered arrangement along one of the paired longitudinal edges 5a, 5b, that is, the longitudinal edge 55a (hereinafter referred to as "first longitudinal edge") which is closer to the heating resistor 52. Each of the output pads 61 is wire-bonded to the above-mentioned other end of a respective individual electrode 5a. The drive IC 55 is provided with a multiplicity of signal pads 62 and power pads 63 which are arranged along the other longitudinal edge, that is, the longitudinal edge 55b (hereinafter referred to as "second longitudinal edge") which is closer to the printed circuit board 56. The signal pads 62 and the power pads 63 are wire-bonded to the connecting portions 57. The drive IC 55 has a drive section 64 and a signal processing section 65 disposed between the output pads 61 and the signal pads 62 or the power pads 63. The drive section 64 is closer to the first longitudinal edge 55a than the signal processing section 65. The drive section 64 is connected to the output pads 61 via wirings and includes a circuit for turning on and off an electrical current passing through the portions of the heating resistor 52 located between the individual electrodes 54 and the teeth of the common electrode 53. The signal processing section 65 is connected to the signal pads 62 by wirings and includes logic circuits as well as a shift register for controlling the drive section 62 by processing the signals which are fed externally via the connecting portions 57.
With such an arrangement, when wire-bonding the signal pads 62 and power pads 63 along the second longitudinal edge 55b of the drive IC 55 to the connecting portions 57 on the printed circuit board 56, it is necessary to cause a flat portion of the upper surface of the printhead substrate 51 to be located under the signal pads 62 and the power pads 63 for bonding wires. To this end, conventionally, the drive IC 55 is die-bonded to the printhead substrate 51 in a manner such that the second longitudinal edge 55b of the drive IC 55 positionally deviates slightly inward from an edge of the printhead substrate 51, thereby ensuring that the signal pads 62 and the power pads 63 are located above the flat portion of the upper surface of the printhead substrate 51. Since a respective printhead substrate 51 is obtained by dividing a master substrate formed with multiple printhead substrates 51 along cutting lines, corners of the above-mentioned edge of the printhead substrate may be often damaged during the dividing process, thereby failing to make the upper surface of the printhead substrates 51 to be a complete plane due to the presence of a chipped portion extending for a certain width from the edge. Thus, when the drive IC 55 is die-bonded onto the printhead substrate 51 in a manner such that the second longitudinal edge 55b of the drive IC 55 positionally corresponds to the edge of the printhead substrate 51, the lower surface of the drive IC 55 is to face the incomplete plane of the upper surface of the printhead substrate 51. As a result, the drive IC 55 may be damaged or come off from the printhead substrate 51 due to a shock caused by the wire-bonding at the signal pads 62 and the power pads 63. It is for prevention of such problems that the second longitudinal edge 55b of the drive IC 55 is arranged to positionally deviate inwardly from the edge of the printed substrate 51 by a predetermined amount. Further, when the second longitudinal edge 55b of the drive IC 55 is arranged to positionally deviate inwardly from the edge of the printhead substrate 51, setting the amount of deviation at a rather larger value can make it unnecessary to strictly require accurate positioning of the die-bonding for the drive IC 55, thereby facilitating the die-bonding process.
Recently, since reduction in size and power of recording devices using printhead substrates 51 has been promoted at a fast pace, it is urgently desired to make smaller printhead substrates 51. However, since the second longitudinal edge 55b of the drive IC 55 is arranged to positionally deviate inwardly from the edge of the printhead substrate 51 because of the reasons described above, the length of the shorter edges of the printhead substrate 51 is wastefully made large correspondingly to the amount of the deviation, thereby hindering the reduction in size of the printhead substrate 51.